1. Field of the Invention
The present invention relates to stepper motor drive circuits and particularly to circuits for controlling the flow of current through the windings on the stepper motor cores during the switching of the winding to step the motor.
2. Description of the Prior Art
In the standard operation of stepper motors, each increment of movement from one detent position of the motor to the next involves the switching of flux through the cores in the stepper motor. This is accomplished by switching the current in the winding mounted about said core. Two accepted methods of switching current in the windings involve bipolar and unipolar circuits. In the bipolar circuit, there is a single winding about each core and change in flux through the core is accomplished by switching the current flow in said winding from one direction to the other. A conventional circuit for achieving a switch in current direction in bipolar switching circuits involves the H-driver circuit which will be described in detail hereinafter.
Another conventional method of switching flux in the cores involves unipolar circuits wherein each core is bifilar wound, i.e., the windings about the cores each have a first coil in which current flows to create flux in one direction and a second coil in which current flows to create flux in the opposite direction. Thus, in order to change the flux through the core, one coil is turned on while the other is turned off and vice versa. These bifilar wound switching circuits are well known in the art. U.S. Pat. Nos. 4,072,888 and 4,127,801 are representative examples of the use of bifilar wound switching circuit in the stepper motor art.
In developing, and working with such stepper motor core switching circuits, we have encountered a problem. The switching circuits generally use transistors which are switched on and off in order to switch the current flow in the windings and consequently the flux in the core. The transistors are arranged in some sort of a complementary arrangement wherein one or more transistors controlling a first current path providing a flux in a first direction switched on while the other complementary transistor or transistor providing a current path in the opposite direction are switched off or vice versa. Because of transient charge storage on the transistors involved, there is considerably slower turn off time in each of the transistors than there is a turn on time in their complementary devices. As a result, if one of the switching transistors is turned on simultaneously with the other being turned off, a situation will be encountered every time the transistors are switched wherein, one or both of the switching transistors will be subject to a brief high current surge during the switching period. This results from both transistors being simultanteously on. In ordinary stepper motors, where switching occurs hundreds of times per second, these repetitive surges will damage the switching transistors and significantly reduce their lives.
As will be described hereinafter in greater detail, the present invention solves this problem with sensing means operative during each of the repetitive switching steps in the stepping of the motor which sense the conduction of the current in the path of the switch being turned off. Then, means responsive to such sensing means switch the switching transistor in the path being turned on to then conduct currents through the path.
In this connection attention is directed to the article "Timing Control for DC Motor Driver with Dynamic Braking", authored by J. A. Bailey, R. G. Black, Jr., and V. Espinoza, IBM Technical Disclosure Bulletin, January 1980, Vol. 22, No. 8A, page 3048. This article deals with a circuit for dynamic braking of a D.C. motor wherein the transistor switch for that motor is turned off but the voltage level across that turned-off switch is sensed, and another transistor which shunts the motor during braking to permit the motor discharge is not turned on until the motor driver transistor is fully off. Clearly this article does not relate to the repetitive switching of current through the coils of a stepper motor or any motor, i.e., it deals with the one shot stopping of a D.C. motor when a stop input is made at the designated terminal.